Method of and apparatus for operating at startup and low load a oncethrough vapor generating system



June 2, 1964 LOW LOAD A ONC W. W. SCHROEDTER METHOD OF AND APPARATUS FOR OPERATING AT STARTUP AND E-THROUGH VAPOR GENERATING SYSTEM Filed July 27, 1961 5 47 h VALVE PRESSURE sENsoR 46 4 4?? REGULATOR W2 2e FINISHING 50 V l LEVEL SENSOR SUPERHEATER 52 E;

49 A E: 29 TURBINE PRIMARY mm/u. su PE R HEATER REHEATER FINISHING REHEATER 32 FURNACE WALL 34 TUBES T ECONOMIZER l6 CONDENSER HEATER @F as INVENTOR. WILLBURT w. SCHROEDTER ATTORNEYS United States Patent "ice This invention generally relates to vapor generating systems that operate in either the subcritical or supercritical pressure ranges. More particularly, it relates to vapor generating systems of the forced-flow, once-through type, and is directed toward changing the bypass and spillover sections normally associated with such systems.

. Although vapor generating systems of the once-through type are widely used, such systems generally contain a number of drawbacks that tend to become more and more objectionable as the capacity of a given system increases, To elaborate, a vapor generating system of the forcedflow, once-through type requires a special section to bypass the working fluid around the turbine section of the system during startup to prevent liquid damage to the turbine, since at this time the Working fluid is not in the form of a vapor. Furthermore, such a bypass is generally needed during low load operation of the turbine to bypass an excess amount of working fluid that is circulated through the tubes in the furnace, such excess amount being required for the cooling of these furnace tubes. The total flow in the tubes to provide proper cooling is typically 30 percent of full load flow.

Such systems also generally require a superheater bypass to compensate for startup conditions when a. relatively large flow of working fluid, again approximately 30 percent of full load flow, is circulated to cool the tubes in the furnace. To reduce the heat necessary to produce a. given superheater outlet temperature, a superheater bypass is employed to circumvent most of the flow around the superheater. Further, during restarting operations when the superheater is at or near its operating temperature, a superheater bypass is utilizedto prevent sudden cooling of the superheater due to the circulation of the colder working fluid therethrough.

Also generally required in systems of this type is an auxiliary fluid feeding line, or spillover, that is coupled to the reheating sections of the bypassed turbine apparatus during startup operations to cool the sections, thereby to prevent overheating.

This invention involves the recognition that startup and low load operations of a forced flow, once-through type of vapor generating system may be accomplished without utilizing superheater bypassing and reheater spillover equipment and without endangering any sections of the system, by providing for recirculation of a portion of the working fluid through at least a portion of the tubes in the furnace in combination with a unique form of turbine bypass in which bypassed fluid is utilized for deaerating and then for fluid preheating. If desired, such bypassed fluid may also advantageously be utilized for other purposes, such as in turbine seals, etc., with any excess flow being discharged by way of an outlet line to a condenser.

Pursuant to the present invention, the turbine bypass system described above as modified, and the superheater bypass and the spillover to the turbine reheating sections 3,l35,% Patented June 2, 1954 the tubes and thereby reduces the flow through the superheater to the turbine section to less than 10 percent full load flow. In accordance with the invention, the turbine bypass is greatly reduced in size, and is also employed to supply vapor for preliminary heating and deaerating purposes. Further, during low load operations, the use of the turbine bypass is eliminated, since no excess flow is produced above that permitted to pass to the turbine. The superheater bypass is eliminated, since, with recirculation, the amount of fluid passed to the superheater to produce a given superheater outlet temperature during an initial startup need not be less than the thru flow (not including the quantity recirculated) that leaves the furnace Wall tubes. During a restarting operation, the amount of fluid circulated remains low enough to avoid sudden cooling of the already hot superheater. Finally, the spillover associated with the turbine reheating sections during startup operations is eliminated, since, with recirculation and the consequent reduction in furnace firing, the danger of overheating the reheater tubes is removed.

Although recirculation of a portion of the working fluid through a section of a once-through vapor generating system is a well known technique, it has been employed heretofore almost exclusively to aid in cooling certain fluid carrying tubes of the system and to reduce furnace firing. Recirculation is employed in this invention, however, to effect drastic changes in other sections of the system.

Although the invention has been described generally above, a better understanding of it may be obtained by consulting the following detailed description of a vapor generating system that embodies the principles of the invention. Further reference should be made to the appended drawing which contains a single figure showing in block diagram form such a system.

Referring to the figure, a storage tank 11, containing a working fluid, is coupled to a feed pump 12 which pumps the working fluid at a predetermined pressure to a preliminary heater 14. After being heated in the heater, the working fluid passes through a control valve 15 to an economizer 16. Within the economizer, the fluid is heated by hot gases from a furnace (not shown), and then passes to a mixing vessel 17. The output of the mixing vessel is coupled to tubular furnace walls 19 Wherein the working fluid is heated by appropriate firing of the furnace to raise the temperature thereof. In the case of heating a working fluid at a supercritical pressure, an apparent vapor is generated while the fluid remains in the single phase state, in contrast to the heating of a working fluid at subcritical pressure when the fluid passes from the liquid state to the vapor state.

The output of the furnace walls 159 is connected to a primary superheater 20 and to a stop valve 21. It the system is operating at subcritical pressures, a separator (not shown) may be used to separate the liquid from the fluid flowing from the furnace walls to be applied to the stop valve 21. The stop valve is in turn connected to a recirculating pump 22 which pumps a portion of the working fluid flowing out of the vapor generator through a stop and check valve 24 to the mixing vessel 17. Within the mixing vessel, the fluid withdrawn for recirculation by the pump 22 is mixed with the fluid from the economizer 16, and the mixture is circulated to the furnace Wall tubes 19.

In the primary superheater 2.0, the working fluid not withdrawn for recirculation is now superheated and is passed to a finishing superheater 25 which superheats the fluid to the required operating temperature.

The output of the finishing superheater 25 is coupled through a turbine stop and governor valve 26 to a high pressure turbine 27. The vapor actuates the turbine, and

s is exhausted therefrom through a valve 29 to an initial reheater 3t) and a finishing reheater 31, wherein the vapor is reheated. From the reheater 31, the vapor passes through a valve 32 to an intermediate pressure turbine 36 which is actuated thereby. The vapor then passes through a valve 35 to low pressure turbines 36, and is exhausted therefrom into a condenser 37 wherein the vapor is condensed to a liquid.

The liquid in the condenser 37 is pumped by a feed pump 39 to a heater 4% for preliminary heating. It then flows to a deaerator 41 for the removal of air or other gases therein, and finally passes to the storage tank 11 to commence another cycle in the vapor generating process.

The turbine bypass apparatus, to be used during startup operations, comprises a bypass valve 42 which couples the output of the finishing superheater 25 to a liquid and vapor separator 44. The valve 42, which is adjusted by a regulating mechanism 45 that is under the control of either one of a pair of pressure sensing devices 46 and 47, throttles the vapor passing therethrough. If the system is being operated at a supercritical pressure, the Working fluid is throttled to a subcritical pressure.

Liquid from the separator 44 flows through an adjustable valve 49 to the condenser 37 where it is added to the liquid condensed from any vapor which may be passing from the low pressure turbines 3d. The liquid level in the separator 44 is regulated by adjusting the valve 49, which is done automatically by a level sensing device that actuates a regulating mechanism 51 coupled to the valve 49.

Vapor from the separator 44 is withdrawn in a line 52 and applied to an excess vapor valve 54, a deaerator valve 55, and a heater valve 56. Each of these valves is adjusted by associated regulating mechanisms 5'7, 59, and 63, respectively, which are in turn controlled by pressure sensing devices 61, 62, and 64, respectively.

Vapor passing through the heater valve 56 is applied to the heater 14 and serves to heat the working fluid that is pumped through the heater coils. The vapor is condensed to a liquid and is pumped out of the heater by a pump 65 which returns the fluid to the deaerator 41.

The vapor passing through the deaerator valve 55 is applied to the deaerator 41 to be used for deaerating the working fluid applied from the heater 4% and the pump 65.

The vapor passing through the excess vapor valve 54 is applied to a de-superheater 66. Liquid from a source (not shown) is applied through an injection valve 67 to the de-superheater 66 wherein it is sprayed into the vapor from the excess vapor valve 54, thereby cooling the vapor. The amount of liquid sprayed into the desuperheater is controlled by a regulating mechanism 69 that is, in turn, controlled by a temperature responsive device '70 which measures the temperature of the vapor passing out of the de-superheater 66. This cooled vapor is applied to the condenser 37 to be added to the liquid condensed from the vapor exhausted from the low pressure turbines 36.

The three pressure sensing devices 61, 62 and 64 are so set that, at a first vapor pressure in the line 52, the deaerator valve 55 opens, under the action of the pressure sensing device 62 and the regulating mechanism 59, admitting vapor to the deaerator 41. As the vapor pressure in the line 52 increases, the sensing device 62 and the mechanism 59 cause the valve 55 partially to close to maintain the pressure in the deaerator at the first pressure. At the same time as valve 55 begins to close, or at a second, higher pressure in the vapor line 52, the valve 56 opens in response to impulses received from valve 55 to admit the excess vapor to the heater 14. As the vapor pressure in the line 52 increases, the sensing device 64 and the mechanism 69 may desirably regulate the valve 56 so that the pressure in the heater 14 rises to, but does not exceed, a third pressure. At a fourth, even higher pressure in the line 52, the excess vapor valve 54 opens under the action of the pressure sensing device 61 and the regulating mechanism 57 and passes to the condenser 37 any vapor, at the fourth pressure and higher pressures, that is not used in the heater 14 and the deaerator 41. As may be seen, then, the bypass system supplies vapor for deaerating and heating, in that order and at two different pressures, respectively, during startup operations, and any excess flow is passed to the condenser 37.

Since, with recirculation, the startup flow of Working fluid is greatly reduced over that which it would be without recirculation, the size of the turbine bypass system is greatly reduced, and a superheater bypass is eliminated altogether. Further, because of furnace firing reductions that come about as a result of the smaller startup flow of working fluid, a spillover line to the reheaters is also eliminated.

Operation of the system during a startup is as follows. The turbine valve 26 is completely closed, thereby effectively removing the turbines 27, 34, and 36 and the reheaters 30 and 31 from the fluid system. Next, the bypass valve 42 is set to automatically open at a given pressure and to maintain such pressure. The feed pumps 39 and 12 are energized, and the valve 15 is opened to circulate less than 30 percent, and preferably less than 10 percent, of full load flow of working fluid through the system. The bypass valve 42 will now open and pass the flow established by valve 15. The recirculating pump 22 is energized, and the working fluid is recirculated through the furnace wall tubes 19. The bypass valve 42, presently under the control of the pressure sensing device 47, provides a predetermined pressure at the outlet of the finishing superheater 25.

At this time, the furnace (not shown) is fired to provide a heat input that is less than 30 percent, and preferably less than 10 percent, of full load firing. As the fluid reaches the desired temperature, at the outlet of the finishing superheater 25, the working fluid is throttled in the bypass valve 42 and passes to the liquid and vapor separator 44. As described above, vapor first passes to the deaerator 41 to deaerate at a constant pressure the working fluid. As the vapor pressure in the line 52 increases, the valve 55 begins to close while at the same time the valve 56 is opened and the excess vapor is applied to the heater 14. Finally, as the pressure increases still further, the additional excess vapor in line 52 passes through the valve 54 to the condenser 37.

When the temperature of the fluid flowing from the finishing superheater 25 becomes high enough to ensure that no liquid is formed as the vapor passes through the turbines 27, 34, and 36, the turbine valve 26 is opened. Vapor now flows to the turbines and to the reheaters 30 and 31. When this occurs, control of the bypass valve 42 is transferred to the pressure sensitive device 46 which operates to reduce the vapor pressure in the line 52 sufficiently to close the excess vapor valve 54 under the action of the pressure sensitive device 61; however, the pressure is retained at a high enough level to retain the valves 55 and 56 open, thereby supplying enough vapor to the deaerator 41 and to the heater 14 to satisfy their respective needs. As may be seen, then, at this time the bypass system is supplying only vapor useful for deaerating and fluid heating; none is exhausted through the excess flow circuit.

Next, the supply of working fluid is increased through an appropriate adjustment of the pumps 39 and 12 and the valve 15, and the firing rate of the furnace is increased to preheat the turbines, to actuate the turbines, and then to synchronize them at some predetermined load. At this point, the bypass system is shut down completely by closing the bypass valve 42. Thereafter, vapor is applied to the heater 14 and to the deaerator 41 for heating and deaeration, respectively, from turbine extraction lines (not shown) that Withdraw vapor from the turbines for this purpose.

After closing ofi the bypass system, the recirculating pump 22 is retained in operation until a sufficient amount of working fluid passes through the furnace wall tubes 19 to the superheaters 20 and to cool the fluid carrying tubes (not shown) that are exposed to the hot furnace gases. This occurs when a given load in the system is reached, and at this point the valve 21 may be closed.

From the description of the invention above, it is apparent that modifications and changes may be made without departing from the spirit of the invention. Such modifications and changes should be deemed to be encompassed by the following claims, which are set forth as follows to define the invention.

I claim:

1. In combination with a vapor generating-turbine system that has in series relationship a source of working fluid, pumping means for circulating the working fluid, furnace wall heating tubes, a superheating section, turbine apparatus, and condensing apparatus; means for facilitating startup and load operations comprising recirculating apparatus for recirculating a portion of the working fluid through at least a portion of the furnace wall heatingtubes to maintain a minimum velocity of the working fluid therein, and bypass means connected in parallel relationship with only the turbine apparatus and having a capacity of substantially less than percent full load flow for bypassing the turbine apparatus.

2. Apparatus as recited in claim 1 in which the bypass means has a capacity of less than 10 percent full load flow capacity.

3. A vapor generating system for operation in the supercritical pressure range during startup and low load operations, comprising furnace wall heating means, superheating means, turbine means, condensing means, the four last-named means being connected in closed series relationship to form a vapor generating unit, pumping means for circulating a Working fluid at supercritical pressures through the unit, means for recirculating a portion of the working fluid through at least a portion of the furnace wall heating means to maintain a minimum velocity of the working fluid therein, and bypass means connected in parallel relationship with only the turbine means and having a capacity of substantially less than 30 percent full load flow for bypassing the turbine means.

4. In combination with a vapor generating-turbine system that has in series relationship a source of Working fluid, pumping means for circulating the working fluid, heating means having a vapor input for preliminarily heating the working fluid, a furnace wall heating section, a superheating section, turbine apparatus, condensing apparatus, and deaerating means having a vapor input for deaerating the working fluid; means comprising recirculating apparatus for recirculating a portion of the working fluid through at least a portion of the furnace wall heating section to maintain a minimum velocity of the working fluid therein, bypass means connected in parallel relationship with only the turbine apparatus and having a capacity of substantially less than 30 percent full load flow for bypassing the turbine apparatus, first means coupling the bypass means to the vapor input of the deaerating means when the pressure in the bypass means is at least as great as a first pressure, second means coupling the bypass means to the vapor input of the heating means when the pressure in the bypass means is at least as great as a second pressure, said second pressure being greater than said first pressure, and means coupling the bypass means to the condensing apparatus when the pressure in the bypass means is at least as great as a third pressure, said third pressure being greater than the first and second pressures.

5. In combination with a bypass system that bypasses the turbine section of a vapor generating system, the vapor generating system having working fluid preliminary heating means, working fluid deaerating means, and vapor condensing means; means for utilizing vapor from the bypass system to deaerate and preliminarily heat the working fluid of the system, comprising first means coupling the bypass system to one of the deaerating and pre liminary heating means when the pressure in the bypass system is at least as great as a first pressure, and second means coupling the bypass system to the other of the deaerating and preliminary heating means when the pressure in the bypass system is at least as great as a second pressure, said second presure being greater than said first pressure.

6. In combination with apparatus as recited in claim 5, means coupling the bypass system to the condensing means when the vapor pressure in the bypass system is greater than a third pressure, said third pressure being greater than said first and second pressures.

7. The method of operating at startup a vapor generating-turbine system that has a furnace and, in series relationship, a tubular section of a furnace wall, a vapor heating section, turbine apparatus, and condensing means; comprising the steps of bypassing only the turbine apparatus, circulating substantially less than 30 percent full load flow of working fluid through the system, recirculating a portion of the working fluid through at least a part of the tubular furnace wall section by returning said working fluid portion directly to said tubular furnace wall section from a point downstream of said part and directly to a point upstream of said part so as to maintain a minimum velocity of the working fluid therein, and firing the furnace at substantially less than 30 percent of full load firing.

8. The method of operating at startup a vapor generating system that has a furnace and, in series relationship, a source of working fluid, pumping means for circulating the working fluid throughout the system, a tubular furnace Wall section, a superheating section, turbine apparatus that has at least one re-heating section, and a condenser; comprising the steps of bypassing only the turbine apparatus, circulating substantially less than 30 percent full load flow of working fluid throughout the system at a first predetermined pressure, recirculating a portion of the working fluid through at least a portion of the tubular furnace wall section, firing the furnace at substantially less than 30 percent full load firing, regulating the pressure at the superheater outlet at a desired working pressure, admitting vapor to the turbine apparatus at a predetermined vapor temperature, increasing the firing of the furnace and synchronizing the turbine apparatus at a predetermined load, and discontinuing the bypassing of the turbine apparatus at a preestablished turbine load.

9. The method as recited in claim 8 in which the amount of fluid initially circulated is less than 10 percent of full load flow and the initial firing rate is less than 10 percent full load firing.

10. The method as recited in claim 8 wherein the desired working pressure is within the supercritical pressure range.

11. The method of operating at startup a vapor generating-turbine system that has a furnace and, in series relationship, a tubular furnace wall section, a vapor heating section, turbine apparaus that has at least one reheat section, condensing means, and a plurality of heat exchangers; comprising the steps of bypassing the turbine apparatus, circulating substantially less than 30 percent full load flow of working fluid through the system, recirculating a portion of the working fluid through at least a portion of the tubular furnace wall section, firing the furnace at substantially less than 30 percent of full load firing, and sequentially applying fluid bypassed around the turbine apparaus to different ones of the plurality of heat exchangers to add heat to the working fluid passing through the heat exchangers.

12. In combination with the method as recited in claim 11, the steps comprising admitting vapor at a predetermined vapor temperature to the turbine apparatus from the vapor heating section, and, after vapor is admitted to the turbine apparatus, discontinuing the application of fluid bypassed around the turbine apparatus to the condensing means.

13. The method of operating a bypass system that bypasses the turbine section of a vapor generating system, the vapor generating system having working fluid preliminary heating means, Working fluid deaerating means, and vapor condensing means; comprising the steps of applying to one of the deaerating and preliminary heating means vapor separated from the fluid in the bypass systern when the pressure in the bypass system is at least as great as a first pressure, and applying to the other one of the deaerating and preliminary heating means vapor separated from the fluid in the bypass system when the pressure in the bypass system is at least as great as a second pressure, the second pressure being greater than the first pressure.

14. In combination with the method as recited in claim 13, the steps comprising applying vapor separated from' References Cited in the file of this patent UNITED STATES PATENTS 1,934,667 Hatter Nov. 7, 1933 2,900,792 Buri Aug. 25, 1959 2,989,038 Schwarz June 20, 1961 3,009,325 Pirsh Nov. 21, 1961 3,019,774 Beyerlein Feb. 6, 1962 3,038,453 Armacost June 12, 1962 FOREIGN PATENTS 709,888 Great Britain June 2, 1954 1,064,869 France Dec. 30, 1953 OTHER REFERENCES The Breed Plant Story, pp. 47-78, January 16, 1961, Electrical World.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3, 135 096 June 2 1964 Willburt W0 Schroedter It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5 line 2O before "load" insert 10w column 6 line 8 for "'presure" read pressure lines 58- and 6% for "'apparaus" each occurrence read apparatus Signed and sealed this 29th day of September 1964o (SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST W; SWIDER Attesting Officer 

5. IN COMBINATION WITH A BYPASS SYSTEM THAT BYPASSES THE TURBINE SECTION OF A VAPOR GENERATING SYSTEM, THE VAPOR GENERATING SYSTEM HAVING WORKING FLUID PRELIMINARY HEATING MEANS, WORKING FLUID DEAERATING MEANS, AND VAPOR CONDENSING MEANS; MEANS FOR UTILIZING VAPOR FROM THE BYPASS SYSTEM TO DEAERATE AND PRELIMINARILY HEAT THE WORKING FLUID OF THE SYSTEM, COMPRISING FIRST MEANS COUPLING THE BYPASS SYSTEM TO ONE OF THE DEAERATING AND PRELIMINARY HEATING MEANS WHEN THE PRESSURE IN THE BYPASS SYSTEM IS AT LEAST AS GREAT AS A FIRST PRESSURE, AND SECOND MEANS COUPLING THE BYPASS SYSTEM TO THE OTHER OF THE DEAERATING AND PRELIMINARY HEATING MEANS WHEN THE PRESSURE IN THE BYPASS SYSTEM IS AT LEAST AS GREAT AS A SECOND PRESSURE, SAID SECOND PRESURE BEING GREATER THAN SAID FIRST PRESSURE. 